The present disclosure is directed to a motor controller and in particular a motor controller for use in a downhole logging tool. This motor controller finds great use in downhole logging tools, and provides a motor controller for a three phase motor incorporated in the logging tool which delivers controllable torque with a more efficient power transfer of electrical power furnished at the surface through the logging cable to the logging tool which incorporates the motor.
Electric motors are included often in downhole logging tools. As wells become deeper, the power requirements of the electric motors in such logging tools become more severe. In general terms, deeper wells suggest the requirements of greater power and yet the logging tool must have a reduced diameter. One helpful step has been to advance from a single phase motor to a three phase motor. As diameter of the logging tool decreases, the size of the motor is constricted, motor design becomes limited. To overcome some of these limitations derived from size and space constraints, it is necessary to increase the AC frequency of the power applied to the motor.
In shallow wells, an increase from 60 hertz to 400 hertz can be tolerated with no loss. As the frequency increases, the net power loss to capacitance in the cable increases. Fortunately, if the well is shallow that is not much of a problem. However, even if the well is shallow, often it will be serviced with a tool which is connected to the end of a very long logging cable while most of the cable remains spooled on a storage drum. Accordingly, even a shallow well may engender certain problems, mainly, the fact that it must be serviced by a long logging cable. The power loss in the logging cable thus becomes a significant factor. An increase from 60 to 400 hertz for the power supplied to the logging cable encounters more than a six fold increase in power loss in the logging cable. While the logging cable can be designed so that capacitance per unit length is reduced, it nevertheless cannot be sufficiently reduced to overcome the six fold increase with a change in frequency to 400 hertz.
One approach is to utilize direct current. This is shown in U.S. Pat. No. 3,887,898 which delivers a direct current along the logging cable. Then, an inverter is used to enable power conversion at the logging tool. There is however limited control capacity available so that the motor which is operated by this equipment can be varied in operation.
The disclosed apparatus provides a system which is far more flexible than that set forth in the referenced patent. This system includes a control system able to control the motor in a variety of situations. This is a marked advantage over the control system set forth in U.S. Pat. No. 4,734,634 which suggests control variation to pick up control of a motor after a momentary inverter interruption of the sort typically occasioned by power loss. The present disclosure thus sets forth a feedback based motor controller which provides variable speed and feedback control as a function of power which is a marked advance over the 1979 application notes of Siliconix.
The present apparatus can be summarized as a three phase power inverter which can be used in a logging tool and which can be powered by either DC or AC power at a relatively low frequency to enable power transmission along the logging cable without excessive power dissipation. The inverter converts the applied DC or low frequency AC into AC having any frequency. Moreover, the conversion to the selected frequency for AC operation of the motor is subject to local control. This is particularly helpful because the typical motor performance relationship of speed versus torque comes into play and can be uniquely handled. That is, the motor in the logging tool is typically used to provide drive to a motorized, pad mounted device. The pad mounted device normally is retracted against the body of the logging tool and is extended outwardly to an opened position. When the pad first moves, there is no resistance encountered and torque requirements are thus low while speed requirements are high. For this moment, the motor is ideally operated at a higher frequency to advance at a higher speed. The arms move outwardly until they contact the surrounding wall of the borehole at which time motor speed should be reduced while providing maximum torque. The torque should increase so that the arms are adequately forced against the sidewall. This involves a feedback system regulating speed versus torque.